In a typical analog cellular telephone system, a plurality of contiguous cells, each having a different assigned set of transmission frequencies, are arranged with handoff means for maintaining continuous communication with mobile telephones moving from cell to cell. As a mobile unit travels along a path that passes from one cell to another, a handoff occurs which switches the mobile unit from a frequency in the set assigned to the cell it is leaving, to a new frequency in the set assigned to the cell it is entering. The handoff action is controlled by a mobile telephone switching office (MTSO) which receives a handoff command or instruction. The handoff command is typically generated when the signal received from the mobile telephone falls below a preselected signal strength thus indicating that the mobile telephone is at the cell boundary.
In an analog system, each cell in a cellular telephone system operates with a different assigned set of transmission frequencies. As a mobile telephone passes from one cell to another, the handoff signal instructs the cell which the mobile telephone is entering to begin transmitting at a frequency which is different from the frequency which was being transmitted by the cell which the mobile telephone was leaving. A similar procedure is followed when the mobile telephone passes into the next contiguous cell. Sets of assigned frequencies are different for adjacent cells, and such sets are not repeated except for cells that are far enough away from each other so that interference problems will not occur. In the case of systems using identification codes, the identification codes are generally not repeated.
A mobile telephone unit typically contains a control unit, a transceiver, and an antenna system. Each cell site typically is provided with a control unit, radio, a power plant, data-terminals, and-antennas. The MTSO provides coordination for all the cell sites and contains suitable processing and switching means. The MTSO also interfaces with the telephone company zone offices for standard hardwired telephone systems. The communication links between the MTSO and the various cell sites are typically microwave, T carriers, or optical fiber, and carry both voice and control data between the cell sites and the MTSO.
When a user sitting in a car activates the receiver of the mobile unit, the receiver scans a plurality of set-up channels which are designated among the total channels assigned to the cell. Typically, there may be 21 set-up channels out of a total of 416 channels. (The remainder are communication channels.) The receiver then selects the strongest set-up channel and locks on for a certain time. Each site is assigned a different set-up channel. Accordingly, locking onto the strongest set-up channel usually means selecting the nearest cell site. This self-location scheme is used in the idle stage and is user-independent. It has a great advantage because it eliminates the load on the transmission at the cell site for locating the mobile unit. The disadvantage of the self-location scheme is that no location information of idle mobile units appears at each cell site. Therefore, when the call initiates from a standard non-mobile or land line to a mobile unit, the paging process is longer. Since a large percentage of calls originates at the mobile unit, the use of self-location schemes is justified. After a delay, for example, one minute, the self-location procedure is repeated.
To make a call from a mobile unit, the user places the called number into an originating register in the mobile unit, checks to see that the number is correct, and pushes a "send" button. A request for service is sent on a selected set-up channel obtained from a self-location scheme as described above. The cell site receives it, and in directional cell sites, selects the best directive antenna for the voice channel to use. At the same time the cell site sends a request to the MTSO via a high-speed data link. The MTSO selects an appropriate voice channel for the call, and the cell site acts on it through the best directive antenna to link the mobile unit. The MTSO also connects the wire-line party through the telephone company central office.
When a land-line party dials a mobile unit number, the telephone company central office recognizes that the called number is mobile and forwards the call to the MTSO. The MTSO sends a paging message to certain cell sites based on the mobile unit number and a suitable search algorithm. Each cell site transmits the page on its own set-up channel. The mobile unit recognizes its own identification on a strong set-up channel, locks onto it, and responds to the cell site. The mobile unit also follows the instruction to tune to an assigned voice channel and initiate an audible signal to alert the user to the incoming call.
When the mobile user is finished with the call, the hang up turns off the transmitter, and a particular signal (signaling tone) transmits to the cell site and both sides free the voice channel. The mobile unit resumes monitoring pages through the strongest set-up channel.
During a call, two parties are on a voice channel. When the mobile unit moves out of the coverage area of a particular cell site, the reception becomes weak. The present cell site requests a handoff via an appropriate signal, for example, a 100 ms burst on the voice channel. The system switches the call to a new frequency channel or a different cell identification code in a new cell site without either interrupting the call or alerting the user. The call continues as long as the user is talking. The user does not notice the handoff occurrences.
When call traffic in a particular area increases, increased capacity may be generated by reducing the area covered by a particular cell, i.e., creating a microcell. For example, if a cell is split into four smaller cells, each with a radius of one-half the original, traffic is increased four fold. Naturally, the smaller the cell, the more handoffs required in a cellular telephone system for a given capacity.
Although in the proper circumstances, reduced cell size is advantageous, certain problems can arise. Very often when cell size is reduced, for example to a radius of less than one mile, very irregular signal strength coverage will result. This may be caused by buildings and other structures, and can therefore become highly dependent upon the location of the mobile unit. Other problems arise in connection with signal interference. Although some cellular telephone systems have employed several sets of frequencies in a small single cell, in an attempt to improve capacity in that cell, this prevents the reuse of the same frequencies or adjacent frequencies in the neighboring cells. The overall capacity of the system thereby decreases, since the number of available channels in a system is proportional to the inverse of the number of different frequency sets employed.
A cellular telephone system in which an antenna set configuration leads to a more uniform signal coverage contour and lowered interference levels is described in U.S. Pat. No. 4,932,049 issued to Lee. The cellular telephone system comprises cells which contain a plurality of antenna sets arranged and configured to limit propagation of signals substantially to one of a plurality of zones or sectors within the boundaries of the cells. The zones or sectors are substantially less in area than the area of the cell. Transmission at any one frequency (of the assigned set of transmission frequencies for the cell) is confined to the zone or sector wherein the mobile telephone has been assigned to such one frequency. Frequency handoff occurs while the mobile unit moves to a different cell.
In order to optimize the usage of the assigned set of transmission frequencies in a zoned or sectored cell described above, multiple access schemes allowing more than one user to use a communication channel could be implemented in the cell. Multiple access is possible because most users of a voice communication system do not fully utilize the capacity of the communication system. More specifically, a typical user who is allocated a channel in the communication system only actively uses the voice channel for a fraction of its allocated time. As an example, a typical user using a voice channel generally speaks for half of the time and listens for the remaining times. Thus the communication-channel is then left unused for at least half of the time. By appropriate identifying by user time slot or code, bursts or pockets of voice signals for different users in digital systems may be transmitted thereby increasing the user capacity of the system.
Analog multiple access schemes such as analog frequency division multiple access have been implemented in cellular telephone systems. Digital multiple access schemes including digital frequency division multiple access, time division multiple access, and code division multiple access have been developed, and it is anticipated that they will also be implemented in cellular telephone systems. It is advantageous to implement a multiple access scheme using digital means. This is because digital communication typically offers better performance, higher capacity, and lower cost. It should be noted that the applications of digital communication are not limited to communicating digital data. Analog voice signal can enjoy the benefits of digital communication by first converting the analog voice signal to a digital signal before transmission. After the digital signal is received by a receiver, the digital signal is then converted back to the analog voice signal.
One of the reasons for the improved performance in a digital communication system is that the system is more tolerant to noise. This is because a threshold level of noise energy is required to change the state of a digital signal. Thus, the communication is relatively error free if the noise energy of the communication medium is below the required level. In addition, it is possible to implement error detection and correction algorithms which further reduce the error rate even if the communication medium is relatively noisy. As a result, it is possible to set up communication channels under noisy environment thereby increasing the capacity of the communication system.
Another reason for the improved performance is that digital data can be easily manipulated using digital processors. Thus, many operations which are difficult to implement using analog means can be implemented using low cost microprocessors and digital logic circuits.